In this description, the gearshift being referred to in particular is the rear one. Such a gearshift moves the chain between the different sprockets of the sprocket assembly associated with the rear wheel of the bicycle.
In addition to the function of moving the chain correctly, the rear gearshift performs the function of keeping the chain under tension correctly when it is engaged by any of the different sprockets and during the gearshifting steps, so as to prevent the chain from dropping.
The rear gearshift typically comprises a first body associated with the bicycle frame and a second body that supports a rocker arm. The latter comprises an inner plate, an outer plate and a pair of toothed wheels arranged between the inner and outer plates and configured to engage the chain.
In the rest of the description, the terms “inner plate” and “outer plate”, respectively, are used to indicate the plate of the rocker arm that, when the rocker arm is mounted on the bicycle, faces the wheel of the bicycle and to the plate of the rocker arm that, in the aforementioned mounted condition, is arranged on the opposite side with respect to the wheel of the bicycle.
The second body is connected to the first body through a pair of articulating connecting rods so as to form an articulated quadrilateral actuation linkage. Such a linkage is actuated mechanically by a sheathed cable or electrically by an electric motor.
Following the actuation of the aforementioned linkage the second body is moved with respect to the first body so as to position the rocker arm at the sprocket preselected by the cyclist and engage the chain on such a sprocket.
The rocker arm is rotatably connected to the second body at a predetermined rotation axis.
In the rest of the description and in the subsequent claims, the terms “axial” or “axially” are used to indicate a direction coinciding with or parallel to the aforementioned rotation axis, whereas the terms “circumferential” or “circumferentially” are used to indicate a direction that rotates around the aforementioned rotation axis.
An elastic element, typically a torsion spring, pushes the rocker arm in rotation about such a rotation axis in a direction of rotation that will be indicated hereinafter as “chain tensioning direction”. Viewing the outer plate of the rocker arm mounted on the bicycle frontally, the chain tensioning direction corresponds to a clockwise direction of rotation of the rocker arm about the aforementioned rotation axis.
During gearshifting and/or during travel, especially on an irregular road surface, the rocker arm is subjected to oscillations about the aforementioned rotation axis. During such oscillations, the rocker arm moves in the direction of rotation opposite to the chain tensioning direction, causing a momentary detensioning of the chain and a consequent risk of chain dropping.
In order to dampen the oscillations of the rocker arm it is known to provide a unidirectional damping device between the rocker arm and the second body. Such a device is configured so as to apply a friction force on the rocker arm when the rocker arm is moved about the rotation axis in the direction of rotation opposite to the chain tensioning direction, and on the other hand to allow the rocker arm to rotate freely in the chain tensioning direction.
Examples of unidirectional damping devices are disclosed in documents US 2013/0203532, US 2012/0083371, US 2013/0310204, US 2016/0176478, US 2013/0288834, US 2014/0371013 and US2009/0054183.
These unidirectional damping devices comprise an unidirectional roller bearing arranged coaxially to the rotation axis of the rocker arm.
The Applicant has observed that the roller bearings used in the unidirectional damping devices described in the aforementioned prior art documents are very bulky. The Applicant has also observed that, in order to avoid a risk of deformation of the housing tracks of the rollers, the maximum torque transmissible through the roller bearings is very limited.
The problem at the basis of the present invention is to make a bicycle gearshift that comprises an unidirectional damping device that is more compact and efficient with respect to those described with reference to the prior art.